The present invention relates to turbomachinery and more particularly, to a bucket cover having an aperture structure for mounting to a turbine blade. In turbomachinery equipment, bucket covers are connected to the top ends of turbine blades or buckets to dampen vibration and prevent twisting of the blades. The bucket covers work with a seal to keep gas or steam from leaking away from the turbine blades. Some bucket covers are integrally formed with the turbine blades. Other bucket covers are small flat plates attached to the end of the bucket by reception of a tenon on the bucket through an opening in the cover and deforming the tenon to form a button on the outer surface. During operation of the turbomachinery equipment, a centrifugal force tends to pull the bucket covers away from the top end of the turbine blades, thus it is critical that the bucket cover be securely fastened to the tenon so that the cover stays connected for long periods of operational cycles.
Each turbine blade or bucket has an upwardly projecting tenon that is received in an aperture of a mating bucket cover. Once a cover is fitted onto the tenon, the bucket cover is attached to the turbine blade by peening or deforming the tenon to form a button head on top of the aperture to secure the cover to the blade similar to a rivet. In the manufacture and maintenance of turbine equipment, manual peening lengthens the cycle time to attach the bucket covers to the turbine blades. FIG. 1 shows a partial sectional view of a conventional deformed xe2x80x9cbuttonxe2x80x9d tenon and a bucket cover 1. The excess deformed material as a result of peening the tenon remains as a rounded button 2 on the top surface of the bucket cover. The opening 4 in the conventional cover 1 is straight-sided and provides no structure to engage the deformed tenon. Also, conventional xe2x80x9cbuttonxe2x80x9d tenon and cover configurations did not provide an optimum steam sealing surface. In addition, during operation, the excess material erodes away due to solid particle erosion within the turbine. The solid particle erosion and the centrifugal force tends to weaken the conventional button cover fastening arrangement shortening the cycle time between replacement and inspections.
FIG. 2 shows a partial sectional view of a another prior art connection between a bucket and its cover 1. One proposed solution to the solid particle erosion problems of button assemblies was to provide a xe2x80x9cfoxholexe2x80x9d or recess 6 in cover 1 to accommodate the button 2. The xe2x80x9cfoxholexe2x80x9d bucket covers must also be manually peened thereby lengthening manufacturing time. Further, opening 4 has a straight sidewall that provides no structure to help retain the deformed tenon. In addition, this foxhole arrangement is still affected by solid particle erosion by providing sharp angular areas where solid particles can collect. The solid particle erosion and the centrifugal force still tends to weaken the xe2x80x9cfoxholexe2x80x9d cover fastening arrangement. While, the cycle time between replacement and inspections might have been slightly improved with this design, any further improvement in cycle time is advantageous.
Thus, what is needed is an apparatus and method of securing a bucket cover to a turbine bucket so that the attachment is secure, assembly efficient and cycle time between periodic maintenance inspections lengthened.
Briefly, in one embodiment of the present invention, a flush bucket cover addresses the drawbacks of the conventional covers by providing structural modifications, which result in both operational and manufacturing advantages. The aperture in the inventive bucket cover has a complex geometry compared to the straight-sided apertures of the prior art. Broadly, the aperture has multiple surfaces in the sidewall so that when the tenon is deformed to fill the aperture during assembly, the tenon will be structurally engaged to the sidewall surfaces of the aperture. More specifically, moving from the outer surface of the cover toward the inner surface which is in contact with the bucket, the aperture has a relatively large outer opening with a periphery defined by a curved shoulder. A beveled surface transitions the sidewall to a vertical throat which has a size corresponding generally to the tenon size, and another beveled surface transitions from the vertical throat to the inner surface of the cover. Seen in cross-section, the aperture resembles a chalice shape with the larger mouth of the chalice shape defining the outer surface of the aperture, and the narrowest throat of the chalice shape defining the size of the tenon. To assemble the cover to the bucket, a tenon of the bucket is received in the aperture with a portion of the tenon extending above the outer surface of the cover. The protruding portion of the tenon is worked by peening to deform the entire tenon to fill the aperture. In this way the tenon takes on the shape of the aperture. The deformed tenon is thereby engaged to multiple surfaces of the aperture to secure the cover to the bucket.
In a completely assembled bucket and cover, a turbine bucket has a deformed tenon extending from a radial tip of the turbine blade and a bucket cover portion attached to the radial tip. The bucket cover comprises an aperture having an engageable geometry to retain the deformed tenon having a mating geometry that fills the aperture. Thus the bucket cover is fastened to the turbine bucket by the aperture and tenon structure.
A method of assembling a bucket cover to a turbine bucket for turbomachinery equipment, comprises the steps of: fitting a bucket cover with an opening having a plurality of sidewall surfaces onto a tenon of a turbine bucket so that a portion of the tenon extends above an outer surface of the cover; applying compressive force to the tenon so as to deform the tenon and thereby fill the opening of the cover and forming a button head on the outer surface of the cover; removing the button head and a planar portion of the cover to provide an assembly in which the tenon and the bucket cover are flush with one another.
The present invention thus provides a cover or shroud for turbine buckets, which has an aperture with an engaging structure for retaining a deformed tenon of a bucket. This structural attachment of the aperture-tenon connection provides a secure connection that is more resistant to centrifugal forces than a conventional button head attachment. In addition, the structure of the aperture-tenon connection also enables the button head to be removed so as to eliminate solid particle erosion problems. These advantages result in a more reliable attachment, which increases the time between maintenance intervals.